The Mysterious Born-Again Nebula

About seven and one-half billion years from now, our sun will have converted nearly all of its hydrogen fuel into helium through fusion, and then burned most of that helium into carbon and oxygen. It will have swollen to a size large enough to fill the solar system nearly to the current orbit of Mars, and lost nearly half of its mass in winds. At this stage a very hot stellar remnant will ionize the ejected material, lighting it up and causing it to glow as a "planetary nebula," a class of objects called by this name not because they are planets, but because they surround their remnant star.

This concise summary of the future sun describes a scenario that, in its details, is thought to be quite accurate. Nonetheless, astronomers trying to refine the models and improve their understanding of what happens in a star's old age find some striking puzzles. The aged star is by no means dormant, for example. Eighty-nine years ago, the planetary nebula star Abell 58 suddenly increased in brightness by a factor of 100. Astronomers today see a bright knot of material near the center of the nebula and believe it is fresh stellar material that was ejected in that outburst; meanwhile the star has wrapped itself in a thick cocoon of dust. Since the star is old, astronomers expected that the knot produced in this born-again event would be deficient in oxygen but rich in carbon, as is found in similar knots in most other planetary nebulae. Instead they found the opposite.

SAO astronomer and post-doc Barbara Ercolano, together with five of her colleagues, used ground-based optical spectroscopy coupled with their new model to analyze the composition of this mysterious knot. They report confirming that the knot is indeed strangely rich in oxygen, but discover that inside the knot is a core of comparatively cold material that is deficient in oxygen; its low temperature had led previous analyses to a mistaken conclusion. Although they cannot yet explain the origin of the excess oxygen in the outer shell, they speculate that this might be because the central star is actually a binary star. Their result, if correct, implies that our sun will in fact not produce this kind of planetary nebula, and sheds new light on the subtleties of a star's senior years.